Not necessarily.  Many contractors will encourage you to replace all the siding on the house.  You can however, replace individual pieces of siding or sections of siding without having to replace all of it.

Usually the lower siding boards get the most weather.  They are exposed to more rain and more sun than the siding boards up closer to, and shielded from the soffits or overhangs.  If a siding board is swollen or soft then it should be replaced.  The first indicator of the siding going bad is on the chimney.  Because the chimney receives very little heat from inside the house it has a tendency to have more moisture inside and therefore weathers the siding more quickly.  If the chimney siding starts to go bad it may be only the upper portion of the chimney, above the roofline, that shows weak first in that section and can be replaced.  It is not uncommon to replace all the siding boards on the chimney with Hardy Siding.  When you look at the siding on the house you need to look underneath the bottom drip edge or weather bead of the siding board.  If looking at the bottom you begin to see fractures or cracks in the bottom of the edge then you have to check see if the board is soft.  This is one of the main problems that Georgia-Pacific and Louisiana-Pacific siding has.  Due to the fractures in the bottom edge of the siding it will absorb rain or beads of water that hang on the bottom drip edge of the siding.  That will cause the siding to continue to swell and rot.  If the siding has these fractures and is still solid you can use a thick primer or even a caulk that is wiped off to help fill these small fractures so that it will absorb less moisture.  This process extends the life of the siding.  If there are no fractures, no soft siding and no swelling there is no reason to replace the Georgia-Pacific and Louisiana-Pacific siding.

Another problem that occurs but less frequently is that Georgia-Pacific and Louisiana-Pacific siding appeared to have nail heads that were sunk too deeply by the nail gun of the contractor.  That does not happen occasionally but generally you observe that the nails have remained in position and that because the siding has swollen at the bottom edge, there are holes where the nail head was.  This is easy to remedy if the siding has not swollen too much by simply filling the nail head with caulk and wiping off excess caulk.

Another less frequent problem with the Georgia-Pacific and Louisiana-Pacific siding was that it would sometimes spall of the surface of the siding board.  This appears as if small chips of the siding has occurred.  Again this problem can be solved by sanding lightly by hand and then using a good acrylic primer.  This is not a long-term solution but generally will get you through another painting season.

You can do a lot of single boards or sections of the house without having to replace all the siding.  If approximately 30% of the boards need replacement then it may be worth considering full replacement at this point.  The siding boards can be replaced with Hardy Siding.  If you have textured siding, Hardy siding has a similar texture.  It’s not exact but is in most cases difficult to discern the difference from a short distance that there is a slight difference in the texture pattern and also the bottom weather bead edge.